A gas turbine comprises a turbine and a compressor driven by the turbine. In particular, when the gas turbine is provided for a gas-steam power plant, the compressor is of the axial flow type. Commonly, the gas turbine is subjected to varying operating conditions resulting in different aerodynamic flow conditions within the compressor. In order to adapt the compressor performance to different operating demands, it is known to provide the compressor with variable guide vanes. The variable guide vanes are to be pivoted about their longitudinal axis in order to adjust their angle of attack.
Each variable guide vane is provided with a journal at its root, wherein the journal is pivot-mounted in a through hole in the compressor casing. The journal is accessible from outside the compressor casing and comprises a lever to be actuated for pivoting the variable guide vane. All levers are coupled by means of a unison ring arranged concentrically around the compressor casing. The rotation of the unison ring actuates each of the variable guide vane levers simultaneously to achieve a corresponding rotational setting of each variable guide vane within the compressor casing.
Further, the unison ring is provided with slide bearings supported on the compressor casing such that the slide bearings slide in circumference direction on the outer surface of the compressor casing, when the unison ring is turned.
The compressor casing is interiorly contacted by hot gas and therefore heated up and thermally expanded. In particular, when the compressor is subjected to transient operating conditions, the thermal expansion of the compressor is transient. If there is no accommodation of the thermal induced expansion between the compressor casing and the unison ring against the compressor casing, the unison ring would shrink into the expanding compressor casing and nip.
Providing a clearance between the unison ring slide bearing and the outer surface of the compressor casing results in a non-concentric and uneven unison ring movement of the unison ring relative to a casing diameter the unison ring is rotating about. This results in circumferential variations in positioning accuracy of each variable guide vane. Redundant unison ring movement is a result of the clearance between the unison ring slide bearings and the compressor casing surface they run on. Temperature induced expansion differences between the compressor casing and the unison ring result in running clearance variations. Currently, a design must provide sufficient clearance allowance between the unison ring slide bearing and the compressor casing to ensure nip of the unison ring will not occur during maximum temperature variations. By contrast an increased clearance will result in increased levels of non-concentric unison ring movement.
The restricting design consideration is during engine warn up when the compressor casing heats up rapidly relative to the unison ring and also during operation in low ambient temperatures. Clearance variation is also exasperated when factoring in machining tolerances of the assembled components and operational wear between the contacting components.
The clearance is set to ensure that the unison ring does not nip and become tight at extreme conditions. This means that for the nominal constant running condition the unison ring clearance is not optimal. Clearance between the slide bearings of the unison ring and their contact surface, the compressor casing, results in redundant movement when an external force is applied to rotate the unison ring about the compressor casing. This is because the unison ring needs to traverse the distance of the clearance in a vertical and horizontal movement until two slide bearings contact the compressor casing and the force is then transferred into rotation about the compressor casing.
Known turbines use cylindrical roller wheels at three equidistant points at the inside diameter of the unison ring. The rollers are made of a rubber compound which would accommodate some compression as the unison ring and the compressor casing experience differential growth. As an alternative it is known to calculate a clearance allowance for a best fit practice of components based on each compressor stage casing nominal operating temperature against a unison ring temperature of 15° C. and the tolerance stack of these components. This calculated diameter is then machined after the complete ring has been assembled but prior to attachment around the compressor casing. Another method adopted to achieve the calculated clearance is to add or remove shims under each bearing pad as an assembly process.